This invention relates to a method of preparing animal feed pellets and, more particularly, to a method employing a unique pressure cooking step for the mash which becomes the animal food pellets. For many years, various cereal grains, plant and animal proteins, roughage products, liquids, and other miscellaneous ingredients have been mixed together to form a mash which was relatively aerated. The mash normally has been fed from a bin holding the bulk mash through a variable screw feeder to an atmospheric conditioner or cooker. Because of the different ingredients and the amounts thereof in various formulas of animal food, different rates of mash introduction and different amounts of moisture addition were required. Further, when the pelleting mash was conditioned with steam under atmospheric pressure, the different formulations required different temperatures for proper cooking. This resulted in temperatures of the mash exiting from the conditioner varying from 70.degree. to 212.degree. (the maximum attainable) and an added moisture variation of from 0 to 8% (or a total moisture in the mash from 7 to 18%). Therefore, conditioning of the mash for pelleting has been an art as contrasted to a science -- and one which has been difficult to master because of the varied demands of temperature and moisture required by each of the formula combinations of ingredients.
According to the prior art, after the mash had been conditioned, i.e., cooked and/or moisturized at atmospheric pressure, it was introduced into a pellet mill. Pellet mills normally have employed a rotating annular die into which the mash is introduced axially and forced under the action of stationary, idler rollers through a plurality of die orifices. Thus, the mash, under atmospheric pressure, was subjected essentially to a tangential force relative to the movement of the annular die. The conditioned mash traveled at different velocities causing the high velocity, coarse particle mash to be at the lagging side of the die orifice and the slow moving, fine particle mash at the leading side. The high velocity of the mash at the lagging side resulted in poor particle adherence, evidenced by fractures in the issuing square or round rods -- the die output being transversely severed to form round pellets or cubes.
According to the invention, the conditioned or cooking of the mash is achieved at a controlled steam pressure above atmospheric which results in a number of advantages. Not only does it eliminate the guesswork heretofore characteristic of the conditioning step but it results in a mash which is deaerated and considerably more plastic making it possible for the roll and die of the pellet mill to achieve a better "bite", thereby changing the through-put of the ingredients through the die orifices. The results in achieving a substantially uniform consistency across any transverse plane and substantially eliminates the heretofore disadvantageous fractures along the lagging side of the pellet rods.
Other advantages and objects of the invention may be seen in the details of the ensuing specification.